An electric motor includes a rotor and a stator. The rotor is rotatable relative to the stator. The rotor includes a plurality of outwardly extending rotor poles. The rotor is mounted on and is coaxial with a rotor shaft. Generally, the rotor is formed from a plurality of laminations that are secured together.
The stator encircles the rotor and is coaxial with both the rotor and the rotor shaft. The stator includes a plurality of inwardly extending stator poles. The number of stator poles differs from the number of rotor poles. Each stator pole has a winding that is energizable for causing the rotor to rotate relative to the stator.
The windings of the stator are systematically energized to cause the rotor to rotate in a particular direction. Typically, when a respective stator winding is energized, the rotor pole that is nearest to the respective stator winding is drawn toward that stator winding. Thus, by systematically energizing the respective stator windings, the rotor can be caused to rotate in a particular direction relative to the stator.
It is possible for the laminations of the rotor to shift relative to each other. For example, a group of laminations of the motor rotor may shift relative to the remainder of the laminations. As a result, the rotor poles become bifurcated into two axially extending sections that are misaligned angularly from one another. This angular misalignment of portions of the rotor poles will adversely affect motor operation and may, in an extreme case, cause the rotor to rotate in a direction opposite to the intended direction of rotation when the respective stator winding is energized. For example, suppose the rotor pole nearest a respective stator winding is to the left of the respective stator winding. During normal motor operation, the rotor pole rotates to the right upon energization of the respective stator winding. If a group of rotor laminations has shifted relative to the remainder of the rotor laminations and the shifted laminations are located nearer the respective stator winding but to the right of the respective stator winding, upon energization of the respective stator winding the rotor may rotate to the right at a speed slower than expected, may not be rotated at all, or may rotate to the left relative to the respective stator winding. The movement of the rotor relative to the respective stator winding is dependent upon the number and extent to shifted laminations in the rotor. In each of the above scenarios, the motor is malfunctioning.
Such motor malfunction, and in particular the rotation of the rotor in a direction opposite to the intended direction of rotation, may be disconcerting when the electric motor is used in an electric power-assisted steering system of a vehicle. If the rotor rotates in a direction opposite to the intended direction of rotation in an electric power-assisted steering system, the vehicle may turn in a direction opposite to the direction intended by the driver. For example, the driver may rotate the steering wheel in a direction to turn the vehicle to the right and the power assistance from the electric motor may cause the vehicle to turn to the left.
Thus, a need exists for an apparatus to detect a shift of rotor laminations and to prevent the energization of the electric motor if a rotor lamination shift is determined.